Brazable aluminium composite materials have long been known from the prior art. The aluminium composites materials used for brazed connection technology usually have an aluminium brazing layer on the surface which melts during the brazing process so that the component that has the aluminium brazing layer is connected to the connection partner by means of a brazed connection. The other layers of the aluminium composite material are not melted. A corresponding connecting process occurs for example during the manufacture of heat exchangers, which usually have a number of brazed connection joints, for example for attaching the lamellae or the tubes carrying liquid. Until now, sheets of metal equipped with external aluminium brazing layers were used for brazing heat exchangers. However, particularly on the brazed connections provided for an I-shaped butt joint, problems occur owing to an excess of aluminium brazing material. As a result of a local surplus of silicon, the liquidus temperature of the connection partner is severely reduced locally, so that a ‘burning through’ or a reduction in the wall thickness of the component may occur in this area. The risk of a surplus of silicon is present for example on the I-shaped butt-joint brazed connections of the main distributor and the associated water-carrying tubes. Even if no ‘burning through’ of the tube to be brazed takes place during the brazing process, areas of reduced wall thickness may be generated or what is known as a ‘liquid film migration’ effect may occur. With a liquid film migration effect, for example, microstructural defects appear on certain areas of the surface which can be attributed to the diffusion of silicon in the composite material. These areas of the brazed components constitute a problem in relation to the longevity of, for example, a brazed heat exchanger. The normally used external aluminium layers of the composite materials used are aluminium brazing layers, which soften during the brazing process and subsequently solidify again. As a result of this, owing to the flow behaviour of the aluminium brazing layer, mechanical problems may additionally arise concerning a notch effect of the solidified, irregular aluminium brazing layer surface on other components of the heat exchanger. In addition, subsequent coating of the heat exchangers may be problematic owing to the melted and re-solidified outer layer. Also disadvantageous is the fact that the external aluminium brazing layers are covered with an aluminium oxide layer which, during the creation of the brazed connections, can cause problems in relation to the brazed connections' wettability with aluminium brazing material. As a result of this, increased use of fluxes becomes necessary in order to guarantee the quality of the brazed connections. From published US patent application US 2003/0099856 A1, for example, a brazable aluminium composite material is known that consists of a total of five layers, whereby an internal aluminium core alloy layer is provided which is surrounded by two outer aluminium brazing layers on which, additionally, a thin aluminium layer is applied. The thin aluminium layer is designed to ensure that the aluminium solder does not oxidise, so that the use of fluxes can be dispensed with. However, the layer structure of the known aluminium composite material is relatively complex, since at least five aluminium alloy layers have to be provided in total. The outer, very thin covering layer, however, melts during the brazing process, which means that the problems described above concerning the melted surface are not solved. In particular, the provision of very thin covering layers on the aluminium brazing layer places high demands on the production process, so that one must expect high manufacturing costs. From U.S. Pat. No. 4,825,941, the manufacture of a heat exchanger is known which has a main distributor that is brazed to the flat tubes carrying liquid by means of I-shaped butt-joint brazed connections.
Starting from this prior art, it is the objective of the present invention to provide a brazable aluminium composite material which has a simple structure, has good brazing properties for the production of I-shaped butt-joint brazed connections, significantly reduces the risk of ‘burning through’ of brazed-on components and provides adequate mechanical properties. Over and above this, a brazed construction should be proposed.